Furthermore, the invention relates to a method for MR imaging and to a computer program for an MR device.
In magnetic resonance imaging (MRI) pulse sequences consisting of RF pulses and switched magnetic field gradients are applied to an object (a patient) placed in a homogeneous magnetic field within an examination volume of an MR device. In this way, phase and frequency encoded magnetic resonance signals are generated, which are scanned by means of RF receiving antennas in order to obtain information from the object and to reconstruct images thereof. Since its initial development, the number of clinically relevant fields of application of MRI has grown enormously. MRI can be applied to almost every part of the body, and it can be used to obtain information about a number of important functions of the human body. The pulse sequence, which is applied during an MRI scan, plays a significant role in the determination of the characteristics of the reconstructed image, such as location and orientation in the object, dimensions, resolution, signal-to-noise ratio, contrast, sensitivity for movements, etcetera. An operator of an MRI device has to choose the appropriate sequence and has to adjust and optimize its parameters for the respective application.
So-called molecular imaging and diagnostics (MID) is rapidly developing during the last years. MID is sometimes defined as the exploitation of specific molecules for image contrast and for diagnosis. This definition refers to the in-vivo measurement and characterization of cellular and molecular level processes in human subjects and to the analysis of biomolecules to screen, diagnose and monitor the human health status and to assess potential risks. An important prerequisite for molecular imaging is the ability to image specific molecular targets.
At the moment, MR imaging is considered to be one of the most promising modalities in molecular imaging. Therefore, MR imaging is expected to play an essential role in the clinical use of MID for screening, targeted drug delivery and therapy evaluation. Highly sensitive contrast agents have recently been used to allow MR imaging of molecular targets and gene expression. As mentioned above, MRI can visualize the anatomy with good spatial resolution, is applicable to all body regions and will allow reproducible and quantitative imaging. It can also be used for intravascular and needle image-guided drug delivery. MR can also partly assess molecular information, for example through spectroscopy.
It is important to note in this context that in particular 19F MRI has a high potential in the field of MID and also in pharmaceutical research. 19F MRI allows the direct quantification of nano particles, which can be used as contrast agents in MID. These nano particles contain 19F based molecules, such as, e.g., PFOB (perfluoro-octyl bromide). The particles are coated with a functionalized protective and stabilizing lipid layer. Depending on the functional groups on the lipid layer the nano particles will bind to protein markers specific to a disease and accumulate at the sites within the body of the patient where the disease is progressing. The accumulated nano particles will show up as bright spots in a corresponding 19F MR image. For an accurate detection and localization of a disease within the body, a precise determination of the position of the bright spots caused by the accumulated nano particles is required.
However, 19F MRI and contrast agent quantification is frequently complicated by strong chemical shift artifacts induced by multi-line spectra of the 19F nuclear spins with a shift range of around 100 ppm. This problem equally occurs in MRI of other nuclei like 31P or 13C. There are many methods known in the art to deal with these problems, such as line saturation or line selection methods, chemical shift encoding techniques or certain deconvolution and iterative reconstruction methods. But these known methods typically lead to significantly reduced SNR (signal-to-noise ratio), significantly increased imaging time, and/or need complex and potentially unstable calculations during image reconstruction.
So-called Turbo Spectroscopic Imaging (TSI) methods relating to spectroscopic MRI are known in the art (see Jeff H. Duyn et al. in Magnetic Resonance in Medicine, Volume 30, Issue 4, 1993, pages 409-414). These known methods provide a full spectral information for each voxel or pixel location but do not provide a satisfactory solution regarding the above-mentioned MID-specific problems in connection with MRI of nuclei having strong chemical shifts. In MID, a single spin density image is typically required to assess local contrast agent concentrations. An optimum SNR is required for MID applications in order to enable accurate assessment of the distribution of a contrast agent in the examined body. A high SNR is crucial in MID because the SNR determines the resolution (voxel size) of the MR images. On the other hand, the particle size and the amount of contrast agent to be used depend on the SNR. Smaller particles and a high spatial resolution enable the detection of smaller lesions, namely the diffusion into or uptake by smaller structures. Furthermore, a smaller amount of contrast agent does not only reduce the costs of the examination procedure, but also minimizes the risks associated with introducing non-physiological substances into the human body.
Therefore, it is readily appreciated that there is a need for an improved device for magnetic resonance imaging which provides maximum SNR for precise assessment of contrast agent distribution. It is consequently an object of the invention to provide an MR device that enables imaging with significantly reduced complications due to strong chemical shift artefacts. A further object of the invention is to provide an MR device, which permits to keep the scan time, i.e. the time required for acquisition of the MR signals for a complete MR image, within acceptable limits.